An object of the present invention is a device for the filtering of a video signal that can be used notably to carry out the contour correction, or magnification, of an image displayed on a monitor. It can be applied especially in medicine, where it can be used for the processing of digitized images from radiography operations.
When a body to be examined, for example that of a patient, is subjected to a radiography, the X-rays that go through this body are absorbed, to varying degrees, by the cells of this body interposed in their path. The intensity of the X-radiation that reaches a point downline at a detector reveals the nature of these cells. There are known ways of recording the signal revealed by the detector in the form of a video signal and of showing it on a monitor. In practice, to carry out signal processing operations on this signal, it is preferred to pass from a detected analog video signal to a digital video signal with analog-digital converters. For the display, the digital video signal is reconverted into an analog signal (with digital-analog converters).
One of the defects of radiological images is that the X-radiation, in crossing the cells, sometimes gives rise to secondary emissions. These secondary emissions, which too are detected by the detector, constitute degradations of the image since they convey artefacts that make the image fuzzy at different points of the image. Normally, in a radiological image, the big structures, which correspond in practice to low spatial frequencies, are well distinguished. By contrast, the small details, which correspond to high spatial frequencies, are then subdued. In particular, the contours of the structures shown, corresponding to sudden transitions of the image signal, depending on whether the contour pertains to a projection that is plumb with one mass or with another different one, are toned down. Another effect strongly diminishes the contribution of the high spatial frequencies of the spectrum. It arises out of the fact that an X photon received by the detector does not give a luminous point but a spot. Indeed, the incident X photon produces secondary X photons and electrons. The dispersal of these secondary photons and electrons in the scintillator material of the detector produces light photons distributed on a spot having a non-negligible dimension. This light spot is the detected signal. Because of these phenomena, the radiological images all undergo a swift decrease of the amplitudes of the spectral components with the frequency.
It is therefore sought naturally to heighten the contours: it is sought to increase the participation in the video signal of certain spectral components located in the top part of the spectrum. This heightening is obtained by a filtering operation.
The performance of this filtering, however, always comes up against a difficulty specific to video images: normally, the signal of these video images is presented with an interleaving of the lines so as to prevent the scintillation effect which is bothersome when viewing an image. Hence, half frames are distinguished, called even-parity frames and odd-parity frames: they are displayed successively on the monitor, generally at a rate of fifty half frames per second, giving a standardized rate of twenty-five images per second for the total image. The notion adopted here will be the commonly used one of the half frame whereas the exact term would be half image. Normally, to be filtered, the interleaved signal must be converted into a progressive signal.
Indeed, the most convenient filters are the convolution filters. The following is the principle of these filters. In a 2D window of the image, a certain number of contiguous picture elements or pixels are identified and the pixel located at the center of the window is assigned a luminosity that depends on the luminosity of this pixel at the center of the window as well as the luminosities of the pixels contiguous to this pixel in this window. This dependence is expressed by a set of coefficients: in practice, the luminosity at the center is an accumulation of the luminosities of the contiguous pixels weighted by coefficients. The processing operation consists theoretically in having luminosities of the contiguous pixels available and in making them bear this algebraic combination. All the same, access must be had to the contiguous pixels. This is why the signal has to be progressive.
A convolution is normally a combination in which all the pixels of an image contribute with an adequate weighting to the filtered luminosity of each of the pixels of the image. However, to limit the computations and hence the processing time of the computers that implement these processing algorithms, it is assumed that it is possible to limit the domain of convolution to a small number of contiguous pixels. Thus 9-pixel domains are known where, in a 3.times.3 pixel matrix, the luminosity of the central pixel is a function of its 8 neighbors. Similarly, there also exist known 25-pixel domains, corresponding to a 5.times.5 pixel window. Preferably, the matrices are odd-parity type matrices because a center of the window can be defined more easily without its being necessary to offset the image. However, the invention could be applied to even-parity matrices, 4.times.4 pixels for example.
When the video signal of a half frame is delivered, it is of course only the even-parity lines or the odd-parity lines that are available. Consequently, the neighborhood that is obtained naturally is an imperfect neighborhood. In particular, because the lines most contiguous to the central line are not present. In practice, if it is sought, all the same, to filter this signal with a convolution filter, the processing obtained for the contours is different depending on whether these contours have a substantially vertical trace or a substantially horizontal trace in the image. For the horizontal trace, given the fact that the filter naturally uses the pixels directly contiguous to the central pixel of the window, there is obtained a resolution that is twice the resolution that can be obtained in the vertical plane. Indeed, the operation is carried out, in this case, on pixels that are twice as much packed together.
One attempt to overcome this drawback consists in using windows of 5.times.5 five theoretical pixels with, in practice, 5.times.3 real pixels since two lines are absent, and in achieving a symmetrical neutralization, by null coefficients, of the luminosities of the pixels located in columns that are directly adjacent to the center of the window. Experience has unfortunately shown that the result obtained is not good. Indeed, in this case, the highest spatial frequency filtered can only be half of the desired frequency. In practice, convolutions are obtained that are of the same type as the convolutions taken in 3.times.3 pixel domains but with additional constraints that lower the spatial resolution.
Normally, the transfer function of the filter should be a bandpass function to enable the accentuation of the central spatial frequencies corresponding to a given resolution. In the high frequencies, the filter has to be attenuated to eliminate the effects of the electronic noises, and so as not to excessively amplify a signal whose high frequency components have low amplitudes with respect to the noise. This is true for the entire image.
In the invention, in studying the spatial frequency spectra corresponding to each of the half images, it has been realized that a definitely acceptable result is obtained if the video signal, delivered interlaced in the form of a half frame, is subjected quite simply to a filtering operation with a high-pass transfer function on the vertical plane and a bandpass transfer function on the horizontal plane. In practice, the spectral aliasing obtained with the filter gives a result that is slightly less satisfactory than with a 5.times.5 filtering in progressive mode. The result thereof is a slight calculated degradation of the signal-to-noise ratio that is equal to some percentile points and is neither visible nor measurable and is therefore not troublesome. By carrying out a filtering on half frames, given that the recomposition of the image on the screen by interleaving of the half-frames inverts one spectrum in relation to the other, ultimately for the vertical direction there is also obtained a bandpass filtering whose center frequency corresponds to the resolution to be obtained on the vertical plane, which is then preferably the same as the horizontal plane.
This same spectral study has furthermore led to the making of a magnification filtering to show a high-quality magnified image. Indeed, to magnify an image, for example to double it, the method uses a quarter of the initial image (a quarter of the even-parity half frame and a quarter of the odd-parity half frame) and four pixels of the image to be shown are replaced by pixels of the initial image. Rather than taking pixels as such, it is possible to choose to interpolate. Normally, the interpolation is not done in the prior art techniques because, strictly speaking, in a duplicated line of a half frame it is necessary to interpolate the luminosities of the pixels of this duplicated line as a function of the pixels of the same line in the same half frame and pixels of another line in another half frame, with opposite parity, located above or below. In the invention, with a same type of filter, it is also possible to resolve this problem.